The concept of using optical fiber in a hydrophone system is not new. One such system proposed fairly recently has a number of potential advantages as compared to conventional accoustical detectors. A brief description of that system and some of its advantages will follow; for a more detailed description, the reader is referred to a paper by Messrs. J. A. Bucaro; H. D. Hardy and E. F. Carome appearing in the Journal of the Accoustical Society of America; 62, p. 1302, of 1977.
The system proposed in that paper consists of two optical fibers excited by two beams of coherent light. These beams are brought to interfere after transmission through a beam-splitter, some optical couplers and alignment tools. One fiber is immersed in a liquid insonified with acoustic waves. That fiber undergoes slight variations in both its refractive index and geometrical shape in response to local pressure within the liquid, i.e. in response to sound waves. Such variations cause phase modulation of the light beam conducted by the fiber. The phase modulated light beam interferes with a reference beam of light conducted by the second optical fiber which is kept at constant pressure. That interference results in an intensity modulation of the combined beam.
That system has definite advantages in its concept. However, very real practical problems arise when attempting to use it in an operational underwater acoustic detection system. One immediate problem is seen to derive from the extreme sensitivity of the alignment of optical elements to both displacement and relative orientation of the optical fiber, coupling, and decoupling elements. Relative mechanical vibrations, for example, should be kept below 0.1 .mu.m of amplitude, i.e., 0.1.times.10.sup.-6 meters. That is clearly difficult to achieve in the environment of a ship or sonobuoy floating on a wavy sea. Other problems arise in coping with the presence of a long portion or section of optical fiber which is subject to variable hydrostatic pressure conditions; to variations in the stress and drag forces induced by the water and movements of the ship; and so on. These factors can introduce noise into the system, causing interference with the modulation induced by the acoustical pressures (and changes therein) to be measured. To appreciate the potential impact of such sources of noise, note the following. Acoustical fluctuations of pressure in the portion of optical fiber near the surface due to surface waves are near 0.1 bar in magnitude. That is to be contrasted with the fluctuations in acoustical pressure to be measured which are in the range of 0.1.times.10.sup.-6 bar.
The above problems have remained important and have not been resolved in this art. Further evidence of this and of the relative complexities of hydrophone systems proposed recently, using optical fibers, is seen in U.S. Pat. No. 4,115,753 which issued Sept. 19, 1978 to the U.S. Department of the Navy. Thus, areas of difficulty have continued to exist in the design and implementation of optical fiber hydrophone systems.